Irritate a male platypus during breeding season, and you may end up trapped by its stumpy hind legs, threatened by a set of sharp spurs that are armed with venom. The painful poison hobbles male competitors and is a handy defense against pesky humans and dogs. It is also a somewhat odd concoction, as might be expected from a mammal that is famous for its egg-laying, duck-billed weirdness. Platypus venom contains a class of molecules that biologists once thought did not occur naturally outside the microscopic world of bacteria.